This invention relates to an algal culturing reef unit, an artificial reef unit and an artificial culturing and fishing area or field unit. More particularly, it relates to such units adapted for raising algae necessary for spawning and hatching of fishery products such as fishes, shellfishes and crustaceans and culturing and protection of the young fishes, shellfishes and crustaceans, and adapted for providing an environment suitable for raising these fishery products.
Recently, various artificial reef units have been developed and practiced for protection and raising of fishery products such as fishes and shellfishes. These units are intended for allurement of adult fishes and shellfishes for providing an artificial fishing field and are also termed as swarming reef. This type of reef may be realized by submerging used vessels, used cars, waste tires or concrete, steel, stone or synthetic resin materials assembled or molded into boxes or lattices.
On account of the recent tendency towards depletion of fishery resources and the problem of the 200-sea mile territory, there is an increasing demand for developing shore fishing. This has led to the development of the artificial reef unit aimed at raising or culturing fishery resources besides the aforementioned swarming reef. This culturing type reef is intended for protection and raising of larval and juvenile fishes and shellfishes.
For raising these juvenile marine animals, it is necessary to provide an environment suitable for settlement and growth of diatoms and other feed plants. Thus, in distinction from the swarming type reef, the culturing reef needs to be submerged in a shallow sea zone with a depth between 5 and 15 meters where there is sufficient supply of natural light beams. However, because of high waves and fast oceanic currents usually encountered in such sea zone, the culturing type reef is required to be sufficiently resistant to these severe environments.
Thus, the fishes and shellfishes would swim in water for a certain period after hatching and grow in size while grazing on diatoms or other sea weeds or microorganisms. The fishes and shellfishes are called juvenile in this stage of development. Thus the presence of diatoms and microorganisms is imperative for the growth of these young aquatic animals. For growth of diatoms, it is essential to supply a sufficient quantity of oxygen and sunlight. To this end, the culturing reef unit needs to be submerged in a shallow sea zone presenting adverse environments and hence needs to be arranged and constructed to withstand the severe operating environments.
From economic considerations, the reef unit is preferably made of concrete in order to resist the adverse conditions usually met in the shallow sea zone. However, when submerged in sea water, concrete units or blocks emit a strong alkali from their surface (pH value, about 13). This strong alkali thus emitted into the sea water acts to hinder the settlement or growth of green algae or diatoms or other microorganisms and occasionally may even kill them. When dead, these algae are converted into a layer of calcares known as arthrocalcares, this layer falling on the concrete surface and interfering with settlement and habitation of the algae. In order to prevent this, it is necessary to neutralize the strong alkali emitted from the concrete surface. However, there is not known so far a technology effective to meet such needs.
In addition, the basic culturing conditions are different from one aquatic animal to another because of differences in habit, feed and the manner of living. However, it may not be said that these basic culturing conditions have not been considered sufficiently with the known types of the artificial reef units or culturing and fishing reef units.